1. Field of the Invention
The invention relates to a motor control system and, more particularly, to a motor, control system that executes drive control of an alternating-current motor by applying alternating-current voltage converted by an inverter from direct-current voltage stepped up by a converter.
2. Description of Related Art
In a related art, there is known an electric vehicle that includes an electric motor as a driving power source. The electric motor is driven by electric power from a battery to output power. A three-phase synchronous alternating-current motor may be used as the electric motor. The three-phase synchronous alternating-current motor is driven by the application of three-phase alternating-current voltage converted by an inverter from direct-current voltage supplied from a power supply.
In addition, in some electric vehicles as described above, the direct-current voltage supplied from the battery is not directly supplied to the inverter but is stepped up by a buck-boost converter to a predetermined command value and then input to the inverter. it is advantageous to step up a system voltage VH with the use of the buck-boost converter to increase the system voltage VH in this way because it is possible to drive the alternating-current motor at higher torque and higher rotation speed.
Sinusoidal pulse width modulation (PWM) control, overmodulation control and rectangular wave control are known as a control method for the three-phase alternating-current motor. These control methods are selectively switched and used on the basis of a driving condition of a vehicle, a modulation factor (described later), and the like, widely.
For example, Japanese Patent Application Publication No. 2006-311768 (JP 2006-311768 A) describes that, in a motor control system that is able to variably control an input voltage to an inverter, keeps a modulation factor in a predetermined control method at a target value. In the motor control system, the inverter (14) converts a system voltage (VH) into, alternating-current voltage and applies the system voltage (VH) to an alternating-current motor (M1) in accordance with torque control executed by a PWM control block (200). A modulation factor target value setting unit (310) sets a modulation factor, by which a loss in a whole system is reduced, as a modulation factor target value (Kmd#) in the predetermined control method in the inverter (14) of which the modulation factor is not fixed. A modulation factor computing unit (330) computes the ratio of the amplitude (Vamp) of a motor required voltage to the input voltage to the inverter (14), that is, the system voltage (VH), to obtain an actual modulation factor (Kmd). A voltage command value generating unit (340) generates a voltage command value (VH#) of the system voltage (VH) on the basis of a comparison between the actual modulation factor (Kmd) and the modulation factor target value (Kmd#). A converter (12) variably controls the system voltage (VH) on the basis of the voltage command value (VH#).
As in the case of the motor control system described in JP 2006-311768 A, in a motor control system that includes a converter, an inverter and an alternating-current motor, it is advantageous to decrease a voltage stepped up by the converter to operate the alternating-current motor in rectangular wave control, that is, so-called single-pulse control, in order to reduce a switching loss in the converter and the inverter. However, because rectangular wave control is voltage phase control under field-weakening control, a motor loss increases as a field-weakening current increases: On the other hand, when a voltage stepped up by the converter is increased to operate the alternating-current motor in sinusoidal PWM control, a motor loss is reduced. However, due to a switching, loss resulting from an increase in the number of switching operations, a loss in the converter and the inverter increases. Thus, a loss of the whole system that includes the alternating-current motor is minimized when the current vector of motor current is on an optimal current advance line at which maximum torque is output or near the optimal current advance line during rectangular wave control.
When the operation of the alternating-current motor is controlled in a rectangular wave control mode in which the current phase of motor current is on the optimal current advance line or near the optimal current advance line in this way, a modulation factor in rectangular wave control is constant (for example, 0.78) and therefore, it is not possible to variably control the system voltage while setting a modulation factor as a target as described in JP 2006-311768 A.
In addition, when the above-described system voltage variable control is applied to a system in which a plurality of alternating-current motors are connected to a single converter via respective inverters in parallel with each other, the rotation speeds and command torques of the respective, motors are usually different from each other and therefore, system voltages at which losses of the respective motors are minimum are generally different from each other. Therefore, in the system that variably controls the system voltage so that the current vectors of motor currents flowing through the alternating-current motors and modulation factors are fed back to minimize a loss, it is necessary to select any one of the motors and execute feedback control such that a loss of the selected one of the motors is minimized.
However, in the case where feedback loops of the current vectors, or the like, are respectively provided in correspondence with the individual motors, when the motor and a feedback control deviation are not appropriately selected, the plurality of feedback controls interfere with each other and as a result, variable control of the system voltage may be unstable or the system voltage may become stepwise, which hinders smooth variable control.